Method and apparatus for controlling specific gravity in a heavy medium process



PWI I9, 1966 P. w. CHASE ETAL 3,246,750

METHOD AND APPARATUS FOR CONTROLLING SPECIFIC GRAVITY IN A HEAVY MEDIUM PROCESS Flled Nov. 13, 1962 2 Sheets-Sheet l mmi .Emmne W mw April 19, 1966 F` W. CHASE z-:TAL 3,246,750

METHOD AND APPARATUS FOR CONTROLLING SPECIFIC GRAVITY IN A HEAVY MEDIUM PROCESS Filed Nov. 13, 1962 AUTO SIGNAL FROM CONTROLLER N 0 l S N E P S U S SUSPENSION DRA /NED FROM FL O 7' PRODUCT N C l TR wm GA AM MN TS DENS/F/ER SUSPENSION RETURNED TO INVENTORS. PAUL W. CHASE and PUMP 50X /0 LUTHER G. HENDR/CKSON By m/ )M Attorney United States Patent O 3,246,750 METHUD AND APPARATUS )FR CNTRLLING SFECHFEC GRAVITY IN A HEAVY MEDIUM PRCESS Paul W. Chase, Mountain Iron, and Luther G. Hendrickson, Duiuth, Minn., assignors to United States Steel Corporatitmn a corporation of Delaware Fiicd Nov. 13, i962., Ser. No. 236,944 12 Ciaims. (Cl. NP9- 12) This invention relates to an improved method and apparatus for controlling specific gravity in a heavy medium minerals separation process.

ln a conventional heavy medium process, mineral particles are introduced to a vessel which contains a medium of specilic gravity intermediate that of the values and gangue in the mineral. Commonly the medium is a water suspension of a finely divided magnetic substance, such as ferrosilicon or magnetite. If the mineral is an ore, such as iron ore, the values sink while the gangue ioats. The reverse occurs with some materials, such as coal. After the sink and float products leave the separating vessel, the suspension is drained therefrom and recovered for re-use. Next the sink and float products are washed. The wash water, along with a fraction of the suspension drained from the products, goes to a magnetic separator and thence to a densifier. The magnetic separator removes nonmagnetic contaminants and the densitier removes water to produce a densied medium of higher specific gravity than that used in the separating vessel. This densiiied medium joins the remainder of the drained suspension and returns to the separating vessel. Water accompanying the mineral particles dilutes the medium in the vessel to the proper specific gravity. Reference can be made to Wade Reissue Patent No. 22,191, dated September 29, 1942, for a detailed showing of a process of this type, although the specific pieces of equipment Wade shows are not of the most modern construction.

To achieve a maximum recovery of the values at the desired grade, the specific gravity of the medium in the separating vessel must be maintained uniformly at a substantially constant value. The usual practice has been for the operator to check the specific gravity of samples of the medium at intervals by manual means, and make any necessary adjustments after each check. Variations in the feed rate, water content of the feed, and densifying characteristics make it ditiicult to exercise effective control by this procedure. Automatic controls also have been proposed heretofore, but those with which we are familiar have disadvantages. For example, it is known to measure the specific gravity of the suspension as it enters the separating vessel and introduce additional water there to control the specific gravity. When water is introduced to thc separating vessel for control purposes, the suspension introduced thereto must be at a specific gravity higher than otherwise desirable to leave room for adjustment. Consequently a larger fraction of the drained suspesnion than otherwise necessary must be treated in the magnetic separator and densiiier. These devices operate at unnecessarily high rates, and the losses of medium may lbe excessive. This form of control also necessitates a measurement of the level of material in the separating vessel. Modern separating vessels usually are of the rotating drum type in which it is difiicult to measure the level of material. The float product discharges over a weir; hence there is little measurable variation in the level.

An object of our invention is to provide an improved method and apparatus for automatically controlling the specific gravity of the medium in a heavy medium minerals separation process and to overcome the disadvantages of previous automatic controls.

A further object is to provide an improved method and apparatus for automtaicaily controlling specific gravity in which we avoid introducing water to the separating vessel for control purposes or taking measurements in this vessel.

A more specific object is to provide an improved automatic control method and apparatus in which the controlled variables include (a) the proportion of the drained suspension going to the densiier, (-b) the densifier screw speed, and (c) the relative height of the densier screw.

In the drawings:

FIGURE 1 is a schematic iiowsheet of a heavy medium minerals separation plant equipped with one yform of our control apparatus;

FIGURE 2 is a schematic wiring diagram of the circuit which controls the height of the densitier screw;

FiGURE 3 is a perspective view of a preferred form of splitter embodied in our apparatus; and

FIGURE 4 is a tlowsheet similar to FIGURE 1, but showing a modification.

F lowsheet FIGURE l shows a fiowsheet of a heavy medium plant which includes a pump box 10, a medium-circulating pump l2., a separating vessel 13, and sink and iioat screens 14 :and 1S. Pump 12 continuously feeds the medium, a water suspension of magnetic particles, from the pump box 1t) to the separating vessel 13. Mineral particles (for example ll/i x 1A inch iron ore) also are introduced to this vessel. Heavier mineral particles sink in the medium and discharge from the separating vessel to the sink screen 14, while lighter mineral particles iloat and discharge to the float screen 15. Screens i4 and 15 have drain sections 14a and 15a and wash sections 14b and 15b over which the sink and iioat products pass in succession. Suspension drained from the sink product as it passes over the drain section 14a returns directly to the pump box llt). Suspension drained from the float product as it passes over the drain section 15a goes to an adjustable splitter 16, which routes variable fractions to the pump box il@ and to another pump -box 17. Water is applied to both the sink and iioat products as they pass over the wash sections 14b and 15b to wash away additional medium. Wash water from both products goes to the pump box 17, from which a pump 1S delivers the contents to a magnetic separator 19 to remove nonmagnetic contaminants. The remaining magnetic particles and water go to a densiiier 2?, which removes water. Densiiied suspension from the densiiier returns to the pump box l0. In the example of an ore, sink particles leaving screen 14 usually are a iinished concentrate product, while float particles leaving screen 1S and nonmlagnetic contaminants from the magnetic separator 19 usually are a finished tailing product.

The individual pieces of apparatus other than our preferred splitter, as well as the portions of the iiowsheet thus far described, lare conventional and hence not described in greater detail. However, we point out that the densiier is of a type in which both the speed and depth of the rake can be changed to increase or decrease the quantity of densiiied suspension feeding therefrom. We have illustrated a densier which has a rotating rake or screw 21 and a motor 22 for lowering or raising the screw. A densiiier of this type, and also a suitable separating vessel and pumps, are available commercially from Western Machinery Co., San Francisco, California, and are described in a printed publication by the supplier entitled Wemco Equipment for Heavy Media Separation, Bulletin No. Htl-B12. Another example of a suitable densier is a reciprocating rake type of classiiier available commercially from Dorr-Oliver Incorporated, Stamford, Connecticut, and is described in a printed publication by 3 the supplier, Bulletin 2281. We also point out that the plant may include other conventional pieces of apparatus, such as demagnetizing coils for the recovered suspension, which we have not shown, since they are not involved in the present invention.

In accordance with our invention, we mount a density meter 23 at 'a convenient place in the owsheet, illustrated in the line which carries medium from pump 12 to the separating vessel 13, and we connect a specific gravity recorder-controller 25 to this meter. The specific gravity of the medium carried in this line is a little higher than that of the' medium actually in the vessel, but it may be considered representative for control purposes. The recorder-controller transmits a signal representative of the density of Ithe medium in vessel 13 to a splitter-positioner 31, which is mechanically connected to the splitter 16. The positioner 31 adjusts the splitter to vary `the fraction of the drained suspension treated in the magnetic separator 19 and densitier 20 inversely with changes in specific gravity. If the specific gravity of the medium entering vessel 13 goes down from its set value, the positioner adjusts the splitter to route a larger fraction of the drained suspension through the magnetic separator and densier, whereupon larger quantities of non-magnetic contaminants and water are removed from the circulating suspension and its specific gravity thus raised. The reverse action occurs if the specific gravity of the medium goes up from its set value.

After an adjustment is made in the splitter position, there would normally be a time lag of several minutes before medium of corrected specific gravity reaches the separating vessel 13, but there is an immediate change in the level of medium in the pump box 10. if a larger fnaction of the drained suspension goes to the magnetic separator 19 and densifier 20, less of course returns directly to the pump box and the level falls from its set value. The reverse action occurs if a smaller fraction goes to the magnetic separator. We connect a levelsensing device 32 to the pump box and connect a controller 33 to the level-sensing device. Controller 33 transmits a signal representative of the level of medium in the pump box to a speed-changing device 34 for the motor which drives the densitier screw 21. When the llevel falls or rises from its set value, the screw immediately commences to turn faster or more slowly. As a result particles actually in transit within the screw feed faster or more slowly to the pump box, and both the specific gravity and the level of the medium in the pump box are corrected promptly. The speed change itself produces only a `temporary change in the feed rate. After the screw has fed all the medium particles actually in transit at the moment of the speed change, the speed change ceases to be factor, but by this time the change caused by adjustment of the splitter takes effect. Hence medium in the pump box continues to be of the corrected specific gravity.

Electric circuit As the plant operates, medium particles gradually are depleted. Hence the trend is for splitter 16 to route a larger and larger fraction of the drained suspension to the magnetic separator 19 and densiiied 20 and for the densifier screw 21 to run faster and faster. We connect the screw speed-changing device 34 with the raiselower motor 22. When the speed of the densier screw 21 approaches a set maximum or minimum, a signal for a further increase or decrease in speed operates motor 22 to lower or raise the screw.

FGURE 2 is a schematic wiring diagram of one form of circuit we can use for operating the raise-lower mo- 7 runs. Timer T1 operates with a maintained contact control switch and we set it to time out after a relatively brief period (for example 8 seconds). Timer T2 operates with a momentary contact control switch and we set it to time out after a longer period (for example 10 minutes). Timer T2 has a normally closed contact 35 and a double-throw contact 36 which are internally connected- We connect line L1 with contact 35. We connect one end of the coils of ltwo parallel latch-type relays A and B to contact 36 through normally open switches 37 and 38 respectively and connect the other end of each coil to line L2. As long as timer T2 is in its off or reset`. position, contact 35 is closed and contact 36 positioned to connect the relay coils with line L1. Switches 37 or 38 closes when the speed-changing device 34 reaches a setting at which the densifier screw 21 is operating at a speed approaching its maximum or its minimum respectively, whereupon relay A or B is energized, provided sutlicient time has elapsed since the previous operation for timer T2 to have reset. Relay A has normally open contacts A1 and A2 and relay B normally open contacts B1 and B2. Whenever either relay is energized, its contacts remain closed even though switch 37 or 38 may open, since the relays are of lthe latch-type.

We connect the timing circuit of timer T1 across contact 36 of timer T2 and line L2 in series with contact A1 or B1, whereby timer T1 is energized and commences to time when either contact A1 or B1 closes. Timer T1 has a normally open contact 39 and a double-throw con-1 tact 40 which are connected internally. We connect line L1 with contact 40 and connect one end of the coils of lower and raise relays 22a and 22b with contact 39 via contacts A2 and B2 respectively. We connect the other ends of the relay coils with line L2. As soon as timer T1 is energized, contact 39 closes. As long as the timer is in its reset position or while it is timing, contact 40 is positioned to connect contact 39 with line L1. Thus whenever relay A is in latch position, relay 22a is energized via contacts 40, 39 Iand A2 to operate motor, 22 in a direction to lower the densilier screw 21. Similarly whenever relay B is in latch position, relay 22b is energized via contacts 4t), 39 and B2 to operate motor 22 in the other direction to raise the screw.

We connect the timing circuit of timer T2 across contact 40 of timer T1 and line L2. When timer T1 times out, contact 40 moves to a position to disconnect and deenergize relay 22a or 22b and to connect timer T2 with, line L1, whereupon timer T2 is energized and commences to time. As soon as timer T2 is energized, contact 36 moves to its other position. We connect parallel reset" coils 41 and 42 for relays A and B across contact 36 and line L2, whereby these coils are energized when contact 36 changes position. The normally open contacts of relay A or B open, whereupon timer T1 is deenergized and resets to the position shown in FGURE 2. Timer T2 continues to run, since it operates with a momentary contact from timer T1. When timer T2 times out, contact 35 momentarily opens and deenergizes the reset coils 41 and 42. Finally timer T2 resets automatically to the position shown in FGURE 2, whereupon the cycle can repeat whenever switch 37 or 38 closes.

Preferably the circuit to relays 22a and 22b includes a manual-automatic selector switch 43 and push buttons 44 land 44a for lowering or raising the densitier screw under manual operation. The circuit can also include limit switches 45 and 45a which open when the screw reaches its extreme positions to prevent further operation of motor 22.

Splitter FIGURE 3 shows structural details of our preferred form -of splitter 16. A first fixed launder 46 receives suspension drained from the iioat product on screen 15. A second fixed launder 47 is located beneath launder 46 to receive the fraction of the suspension which returns directly to the pump box 10. A third fixed launder 43 assenso is located beneath launder i6 and offset therefrom to receive the fraction which goes to the magnetic separator 19 and densier 20. The splitter includes a swinging launder is located in the space between the txed launders and pivoted to an overhead support I. The swinging launder is located in the space between the iixed launders 46 and 47 and it has an outlet spout S2 in its end above the xed launder 4S. The positioner 3l is connected to the links Sil to move the swinging launder back and forth in response to signals from the recorder-controller Z5' (FIGURE l) or controller 33 (FIGURE 4). The splitter construction per se is claimed in another application led by the co-inventor Hendrickson.

FIGURE 4 shows a modiiication in which We connect the specific gravity recorder-controller 25 with the speedchanging device 34, and connect the level controller 33 with the splitter-positioner 3l. When the specific gravity of the medium changes from its set value, the recordercontroller Z5 adjusts the screw speed-changer 3ft to produce an inverse change in the speed of the densier screw ZI. The change in the screw speed changes the volume of material going to the pump box lil. As a result of the change in the screw speed, the level of medium in the pump box Il@ rises when the specific gravity goes down or falls when the specic gravity goes up. Controller 33 transmits a signal representative of the change in level to the splitter-positioner 3i, which adjusts the splitter I6 in a direction to restore the level toits set value. That is, if the level in the pump box 10 rises, the splitter routes a larger fraction of the drained suspension to the magnetic separator I9 and densifier 20, and a smaller fraction directly to the pump box iti. The reverse action takes place if the level in the pump box falls. As in the embodiment shown in FIGURE l, the effect of the screw speed change is temporary, but by the time the medium in transit in the screw has all been fed, the change brought about by adjustment of the splitter becomes eiective. In this manner the speciiic gravity of the medium is corrected to its set value. In other respects the ilowsheet shown in FIGURE 4 is similar to that shown in FIGURE l; hence we have not repeated the description.

Instruments The individual instruments used in our control apparatus are of conventional construction and available cornmercially. Hence We have not shown nor described them in detail, but instead reference can be made to printed publications for showings.

Considine, Process Instruments and Controls Handboo published by McGraw Hill Book Company, copyright 1957, Library of Congress Catalog Card No. 56-8l69, shows and describes instruments suitable for several of our purposes. Considine shows a recordercontroller (page ll-22 or 11-26) suitable for our recorder-controller 25, and a cylinder-type operator with positioner (page 10-37) suitable for our splitter-positioner 3l. A suitable recorder-controller 25 also is available commercially from Leeds and Northrup Company, Philadelphia, Pa., as Type H with CAT controller and is described in printed publications by the supplier Data Sheets ND-4643 (106) 80-558 and ND-46-5l (100) 60-658 and Folder ND-4 (76) Sti-i158 pages 9 and l0. A suitable positioner is available commercially from Foxboro Company, Foxboro, Mass. as the Stabiload, and is described in a printed publication by the supplier, Bulletin No. 446. Our density meter 23 can be an Ohmart Cell, as shown in Ohmart Patent No. 2,763,790, or a gamma gage. A suitable level-sensing device 32 and level 4controller 33 are available commercially from Fisher Governor Company, Marshalltown, Iowa, as Type 249P and Type 2500 Fisher Level-Trol and are described in a printed publication by the supplier, Bulletin F-4A. A suitable screw speed changer 34 is available commercially from Louis Allis Co., Milwaukee, Wis., as the Select-a-speed drive, and is described in a printed publication by the supplier, Service Manual, Section 14A, July l, 1957. Suitable timers T1 and T2 are available commercially from Eagle Signal C0., Moline, Illinois, as the Cycl-Flex reset timer and are described in a printed publication by the supplier, Bulletin 120, August 1955.

Our illustrative recorder-controller 25 generates an electric signal. When we use this signal to control a pneumatically operated positioner 3l, as shown in FIG- URE l, we include an electric-to-pneumatic transducer. A suitable transducer for this purpose is available commercially from Fisher Governor Co. as Type 543 and is described in a printed publication by the supplier, Bulletin E543. Our illustrative level controller 33 generates a pneumatic signal. When we use this signal to control the speed of the densier motor, also as shown in FIGURE l, we include a pneumatic-to-electric transducer. A suitable transducer for the latter purpose is available commercially from Taylor Instrument Companies, as the Transccpe Servomatic Transducer and is described in a printed publication by the supplier, Bulletin 98375, May 1960.

From the foregoing description, it is seen our invention affords a relatively simple yet effective method and apparatus for controlling the specic gravity of the medium used in a heavy medium process. We avoid introducing water to the separating vessel for control purposes, as well as taking measurements in this vessel, thereby overcoming diiculties encountered with previous automatic controls.

While we have shown and described certain preferred embodiments of the invention, it is apparent that other modifications may arise. Therefore, we do not wish to be limited to the disciosure set forth but only by the scope of the appended claims.

We claim:

d. In a heavy medium minerals separation process in which mineral particles are introduced to a Water suspension of magnetic particles, heavier mineral particles sink yin the suspension while lighter mineral particles iloat, the resulting sink and float products are successively drained of `suspens-ion and washed with water, lboth the wash water and a varia-ble fraction of the drained suspension are treated to remove nonmagnetic contaminants and water and `thereby produce a densitied suspension, said densitied suspension together with the remainder of the drained suspension is transferred to a pump box, andthe suspension feeds from the pump box to a vessel where the mineral particles are introduced, the combination therewith of a method or controlling the specific gravity `of the suspension in the vessel to a set value comprising measuring a specic gravity representative of the suspension in the vessel, measuring changes in the level `ot" the suspension in the pump box from `a set level, and adjusting lboth the magnitude of the treated fraction of the drained suspension and the rate at which said densiii-ed suspension is transferred to the pump box in response to these measurements to treat a larger fraction and transfer more of said densified suspension as the specific gravity goes down from said set value and the reverse as the 'speciiic gravity goes up, with one of the `adjusting steps following the other.

2. In a heavy medium minerals separation process in which mineral particles are introduced to a water suspension of magnetic particles, heavier mineral particles sink in the suspension while lighter mineral particles float, the resulting sink and -float products are successively drained of suspension and washed with water, both the wash Water and a variable traction of the drained `suspension are treated to remove nonmagnetic 4contaminants and 'water and thereby produce a densied suspension, said densied suspension together with the remainder of the drained suspension is transferred to a pump box, and the suspension `feeds from the pump box to `a vessel where the mineral particles are introduced, the combination therewith of a method of controlling the specific gravity of the suspension in the vessel to a set value comprising measuring a speciiic gravity representative of the suspension in the vessel, adjusting the magnitude of the treated fraction of the drained suspension to produce more of said densitied suspension as the specific gravity goes down from said set value and less as the specific gravity goes up, measuring changes in the level of the suspension in the pump box from a set level, `and adjusting the rate at which said densitied suspension is transferred to the pump box in accordance with the level measurement to transfer `more of said densiiied suspension as the level falls from said set level and less when the level rises, with .the second-named adjusting step following the rst.

3. In a heavy medium minerals separation process in which mineral particles are introduced to a water suspension of magnetic particles, heavier mineral particles sink in the suspension while lighter mineral particles float, the resulting sink and iioat products are successively drained of Isuspension and washed with water, both the wash water and a variable fraction of the drained suspension are treated to remove nonmagnetic contaminants and water and thereby produce a densiiie-d suspension, said densitied suspension together with the remainder of the drained suspension is transferred to a pump box, and the suspension feeds from the pump box to a vessel where the mineral particles are introduced, the combination therewith of a method of controlling the specific gravity of the suspension in the vessel to a set value comprising `measuring a specific gravity representative of the suspension in the vessel, adjusting the rate at which said densitied suspension is transferred to the pump box in accordance with changes in the specific gravity measurements to transfer more when the specic gravity goes down and less when the specific gravity goes up, measuring changes in the level of the suspension in the pump box from a set level, and adjusting the magnitude of the treated fraction of the drained suspension to produce more of said densied suspension as the level ri-ses and less as the level falls, with the secondnamed adjusting step following the ii'rst.

4. In a heavy medium minerals separation process in which mineral particles are introduced to a water suspension of magnetic particles, heavier mineral particles sink in the suspension while lighter mineral particles iloat, the resulting sink and float products are successively drained of suspension and washed with water, both the wash water and a variable fraction of the drained suspension are treated to remove nonmagnetic contaminants and water and thereby produce a densified suspension, said densified suspension together with the remainder of the drained suspension is Ktransferred to a pump box, and the suspension feeds from the pump box to la vessel where the mineral particles are introduced, thecombination therewith of a method of controlling the specific gravity of the suspension in the vessel to a set Value comprising measuring the specific Gravity of the suspension feeding from the pump box to the vessel, measuring changes in the level of the suspension in the pump box from a set level, adjusting the magnitude of the treated fraction of the drained suspension in accordance with changes in one of the measurements to correct the specic gravity after an interval required lfor suspension of corrected specific gravity to get back to the vessel, and temporarily adjusting the rate at which said densilied suspension is transferred to the p-ump box in accordance with changes in the other measurement to correct the specific gravity promptly until the iirst correction takes effect.

5. A method as defined in 4claim 4 in which the magnitude of the treated fraction is adjusted in accordance with changes in the specific gravity measurement, and

8 the rate at which densilied medium is transferred is adjusted in accordance with changes inthe level measurement.

6. A method as defined in claim 4 in which the magnitude of the treated fraction is adjusted in accordance with changes in the level measurement, and the rate at which said densilied medium is transferred is adjusted in accordance with changes in the specific gravity measurement.

7. In a heavy medium minerals separation plant, which includes a separating vessel adapted to contain a water suspension of magnetic particles in which heavier mineral particles sink while lighter mineral particles float, means for successively draining the suspension from sink and float products from said vessel yand washing the products with water, means `for treating both the wash water and a variable fraction of the drained suspension to remove nonmagnetic contaminants and water and thereby producing a densitied suspension, a pump box to which are transferred said densiiied suspension from said treating means and the remainder of the drained suspension, and a pump `for feeding the combined densitied and drained suspension from said pump box to said vessel, the combination therewith of an apparatus for controlling the specific gravity of the suspension in said vessel to a set value comprising means for measuring a specific gravity representative of the suspension in said vessel, means for measuring changes in the level of suspension in said pump box from said set level, means operatively connected with one of said measuring means for varying the magnitude of the treated fraction of t-he drained suspension in accordance with changes in the measurement, and means operatively connected with the other measuring means for varying the rate at which said den-silied suspension is transferred to the pump lbox in accordance with changes in the other measurement, with the means for varying the magnitude and the Imeans `for varying the rate operating so that one follows the other.

8. A combination as defined in claim 7 in which the means for varying the magnitude of the treated fraction is operatively connected with the means for measuring lthe specific gravity, and the means for varying the rate is operatively -connnected with the means for measuring the level.

9. A combination as dened in claim 7 in which the `means for `varying the magnitude of the treated fraction is operatively connected with t-he means for measuring the level, and the means for varying the rate is operatively connected with the means for measuring the specific gravity.

10. `In a heavy medium minerals separation plant, which includes a separating vessel adapted to contain a water suspension of magnetic particles in which heavier mineral particles sink while lighter mineral particles float, means for successively draining the suspension `from sink and float products from said vessel and washing the products with water, an adjustable splitter for dividing into variable fractions the suspension drained from at least one of the products, means for treating `both the wash water and a fraction of 'the drained suspension to remove nonmagnetic contaminants and water and thereby produce a densifled suspension, a pump box to which are transferred said densitied suspension from said treating means and the remainder of the drained suspension, and la pump rfor feeding the combined densitied and drained suspension from said pump box to said vessel, the combination therewith of an apparatus for controlling the specific gravity of the suspension in said vessel to a set value comprising means for measuring a specitic gravity representative of the suspension in said vessel, means for measuring changes in the level of suspension in said pump box from a set level, means operatively connected with one of said measuring means and with said splitter to adjust the magnitude of the treated fraction in accordance with changes in the measurement to correct the specic gravity after an interval required for suspension of corrected speciii-c gravity to get back to said vessel, and means operatively connected with the other of said .measuring means and with said treating means ifor temporarily 'adjusting the rate at which said densified suspension is transferred to said pump box to correct the specic gravity promptly until the rst correction takes effect.

11. A combination as defined in cl-aim 10 in which the treating means for removing water from the drained suspension includes a densier having a rotatable screw, drive means for said screw, and means rEor raising and lowering said screw, and -in which the means for temporarily adjusting the rate at which said densied suspension is transferred includes means for adjusting the speed of said drive means, to transfer particle-s actually in transit Within said screw at a changed rate.

12. A combination as dedined in claim 411 including means for adjusting said Iraising `and lowering means 10 when large adjustments are needed in 'the rate at which said densi-tied suspension is transferred.

References Cited by the Examiner UNITED STATES PATENTS 2,584,076 1/1952 Wurzback 209-489 2,690,261 9/ 1954 Maust 209-1725 2,690,262 9/1954 Bean 209-1725 2,933,187 4/1960 Old 209-464 3,093,577 6/ 1963 Wilmot 209-1725 FOREIGN PATENTS 136,541 3/ 1950 Australia.

938,831 4/1948 France.

75 3,541 7/ 1956 Great Britain.

HARRY B. THORNTON, Primary Examiner.

HERBERT L. MARTIN, Examiner. 

1. IN A HEAVY MEDIUM MINERALS SEPARATION PROCESS IN WHICH MINERAL PARTICLES ARE INTRODUCED TO A WATER SUSPENSION OF MAGNETIC PARTICLES, HEAVIER MINERAL PARTICLES SINK IN THE SUSPENSION WHILE LIGHTER MINERAL PARTICLES FLOAT, THE RESULTING SINK AND FLOAT PRODUCTS ARE SUCCESSIVELY DRAINED OF SUSPENSION AND WASHED WITH WATER, BOTH THE WASH WATER AND A VARIABLE FRACTION OF THE DRAINED SUSPENSION ARE TREATED TO REMOVE NONMAGNETIC CONTAMINANTS AND WATER AND THEREBY PRODUCE A DENSIFIED SUSPENSION, SAID DENSIFIED SUSPENSION TOGETHER WITH THE REMAINDER OF THE DRAINED SUSPENSION IS TRANSFERRED TO A PUMP BOX, AND THE SUSPENSION FEEDS FROM THE PUMP BOX TO A VESSEL WHERE THE MINERAL PARTICLES ARE INTRODUCED, THE COMBINATION THEREWITH OF A METHOD OF CONTROLLING THE SPECIFIC GRAVITY OF THE SUSPENSION IN THE VESSEL TO A SET VALUE COMPRISING MEASURING A SPECIFIC GRAVITY REPRESENTATIVE OF THE SUSPENSION IN THE VESSEL, MEASURING CHANGES IN THE LEVEL OF THE SUSPENSION IN THE PUMP BOX FROM A SET LEVEL, AND ADJUSTING BOTH THE MAGNITUDE OF THE TREATED FRACTION OF THE DRAINED SUSPENSION AND THE RATE AT WHICH SAID DENSIFIED SUSPENSION IS TRANSFERRED TO THE PUMP BOX IN RESPONSE TO THESE MEASUREMENTS TO TREAT A LARGER FRACTION AND TRANSFER MORE OF SAID DENSIFIED SUSPENSION AS THE SPECIFIC GRAVITY GOES DOWN FROM SAID SET VALUE AND THE REVERSE AS THE SPECIFIC GRAVITY GOES UP, WITH ONE OF THE ADJUSTING STEPS FOLLOWING THE OTHER. 